In recent years, there has been an increasing trend in the field of electronic and electrical devices, for example, toward enhanced performance and reduced size and thickness. Correspondingly, for semiconductor devices provided in electronic and electrical devices for mainly controlling their operations, there is a strong need for such semiconductor devices to be reduced in thickness and also to be formed for dense packaging on printed wiring boards. To satisfy such a need, electronic components, such as chip parts and CSP (chip size package) type semiconductor elements, are being further reduced in size and enhanced in performance. Also, for the printed wiring boards, wiring patterns are being further miniaturized, for example, by reducing wiring widths and pitches between wiring conductors, thereby making it possible to package electronic components on printed wiring boards with higher density.
To achieve denser packaging, some electronic devices employ a recently developed structure in which a plurality of printed wiring boards are stacked and electrically connected via a connector provided therebetween. Specifically, two printed wiring boards to be stacked together have a pair of connectors attached at the centers of their corresponding principal surfaces, and the connectors are coupled so that the printed wiring boards are stacked in the vertical direction of the principal surfaces. In such a stacked structure, most of the load vertically applied to the principal surfaces of the printed wiring boards concentrates at the joint between the connectors, so that stress is caused at the joint. As a result, electrical connections within the connectors might be broken. When further excessive stress is caused, the connectors themselves might be broken. Also, when load is locally applied to an end portion of the stacked printed wiring boards, the gap between the boards is narrowed on the side under load, so that the printed wiring boards or electronic components might be brought into mutual contact. In the worst case, the printed wiring boards or electronic components could be broken. Moreover, even when the applied load is relatively low, if the printed wiring boards are repeatedly subjected to such a load, the boards might be deformed so that connections between the components and the boards are broken.
On the other hand, thermosetting resin compositions are widely used for packaging leadless- or chip-type electronic components on printed wiring boards. For example, a thermosetting resin composition is applied to a predetermined position on the surface of the printed wiring board through selective printing, and electronic components are mounted and attached onto the applied composition before the composition is cured by heating to temporarily joint the electronic components to the printed wiring board. Then, the printed wiring board is fluxed and dipped in molten solder, thereby electrically connecting and fixing the electronic components to the wiring of the printed wiring board. In this manner, the electronic components are completely packaged on the printed wiring board.
However, in recent years, the thermosetting resin composition has come into use for other applications in addition to those as described above, including bonding and fixing electronic components to the surface of the printed wiring board. For example, Japanese Laid-Open Patent Publication No. 2006-049381 proposes a device in which a thermosetting insulating adhesive is applied between a shield case and electronic components packaged on a printed wiring board in order to prevent contact therebetween.
Japanese Laid-Open Patent Publication No. Hei 11-307973 proposes a voltage-controlled oscillator including a printed wiring board, a main voltage-controlled oscillator unit provided with an oscillation coil, and a shield case, in which the oscillation coil is fixed to the shield case via an adhesive, thereby preventing the voltage-controlled oscillator from causing howling. While the adhesive is described as a thermosetting resin composition, the thermosetting resin composition is intended solely for fixing the oscillation coil to the shield case.
Japanese Laid-Open Patent Publication No. 2005-286181 discloses an electronic device including a printed wiring board and a module board attached thereto, in which electrical contacts between the printed wiring board and the module board are elastic connectors. Illustrated examples of the connectors include metallic helical springs and conductive rubbers.
Also, Japanese Laid-Open Patent Publication No. 2005-286181 proposes a device in which a module board is jointed to a printed wiring board by providing an adhesive layer between the module board and electronic components packaged on the printed wiring board. While the adhesive contains thermoplastic resin, the layer is not thick enough to allow sufficiently wide dispersion of stress caused by load externally applied thereto. Accordingly, in the case of the structure described in this publication, as in the aforementioned stacked structure employing connectors, external stress mainly concentrates at electronic components, which might lead to, for example, deformation, breakage, or loss of the electronic components, resulting in poor connections between the printed wiring board and the module board.